The PCI (Peripheral Component Interconnect) bus is a computer system bus that interconnects peripheral components. The PCI bus was developed by Intel Corporation, Santa Clara, Calif., and is an important standard in the computer industry.
PCI devices communicate with each other by sending and receiving electronic signals over the PCI bus. Some (typically older) PCI devices are designed to communicate using signals ranging up to about five volts (5V), while other (typically newer) PCI devices are designed to communicate using signals ranging up to about three volts (3V).
It is desirable to be able to connect both 5V PCI devices and 3V PCI devices to the same PCI bus. However, if both 5V and 3V PCI devices are connected to the same bus, then the 3V devices may be damaged by the 5V signals on the bus. The damage typically occurs when transistor voltage limits are exceeded within the 3V devices.
The transistor voltage limits are limitations specific to the process used to manufacture the integrated circuit (IC) device. These limitations are typically incorporated into design rules by which the IC is designed. Newer and more advanced processes typically have design rules which specify smaller ranges of acceptable transistor voltages. For example, design rules for a newer process may specify that the gate-to-source voltage (Vgs) for a transistor should not exceed 3.0 volts and that the drain-to-source voltage (Vds) for a transistor should not exceed 3.3 volts. Meanwhile, design rules for a previous process may specify that neither Vgs nor Vds should exceed 3.6 volts.